This page only applies to Bitcoin and Litecoin.
By default the BitBoxApp offers you up to 4x different "Priority" options (fee levels) on the page where you create your transaction:
If you want to choose custom fee rate, go to the BitBoxApp settings and enable "Enable custom fees".
When you then make a transaction, you will have another fee level, called "Custom" available.
There you can enter the fee in sat/byte.
To see current fee estimates in sat/byte have a look at https://mempool.space/
If you choose a low fee, it might take a very long time until your transaction gets confirmed.